1. Field of the Invention
The present invention relates to a semiconductor laser drive device, and an image recording device whose function includes scanning a predetermined body to be scanned with a laser beam carrying image information in the course of recording an image.
2. Related Prior Art
In an image recording device employing, for example, a xerography process, a semiconductor laser suitable for a high speed modulation has been used as means for generating an optical beam.
In the case where such a semiconductor laser is employed for an image recording device, there is provided an intermittent feedback control comprising actions of obtaining a control voltage at which a desired light quantity is attained by conducting APC (Auto Power Control) wherein feedback is effected with use of a photosensor monitoring a light quantity in timing not to give influence on formation of an actual image, in order to emit an optical beam with a desired light quantity from the semiconductor laser; then stopping the feedback while the control voltage is held in either an analog mode or a digital mode; conducting an open loop control, wherein a current supplied to the semiconductor laser is set according to the control voltage being held, in timing to give an influence on formation of an actual image; and thereafter the feedback is applied. In modulation of a semiconductor laser, there are a pulse width modulation mode and an intensity modulation mode and techniques are provided for the respective modes in order to improve a quality of an image. What is especially problematic in application of the intensity modulation mode is how to produce an optical beam giving a desired light quantity in a open loop.
Current vs. output power characteristics of a semiconductor laser will be described below and techniques having been conventionally employed will be explained.
FIG. 7 is a graph showing a relation between the current supplied to a semiconductor laser and the emission power of a laser beam output from the semiconductor laser.
As shown in the figure, the optical output characteristics of an semiconductor laser are divided into two regions, one of which is a LED region where a current supplied to the semiconductor laser is less than a predetermined laser oscillation threshold current I.sub.th and the other is a laser oscillation region where a current supplied to the semiconductor laser is the laser oscillation threshold current or more. In the LED region, a laser beam is not emitted from a semiconductor laser and in the laser region, an optical output having a power roughly proportional to a magnitude of a remaining current obtained after a supplied current is subtracted with the laser oscillation threshold current I.sub.th. For this reason, it is necessary to exactly determine the laser oscillation threshold current T.sub.th and an inclination .eta. (laser emission efficiency) in the laser oscillation region. The reason is that it is necessary for a bias current having a magnitude in the vicinity of the laser oscillation threshold current I.sub.th to be made to constantly flow including a time when no laser beam is emitted, in order to secure a high speed exchange between an ON state and off state (see [Hetrostructures lasers] HC CASEY, JR., M. B. PANISH, ACADEMIC PRESS NEW YORK 1978, Japanese Published Unexamined Patent Application No. Sho 60-18982) and there is another requirement to exactly determine a drive current to overlap the bias current in order to obtain a correct quantity of emission. Since these threshold current and inclination are fluctuated in each semiconductor laser and affected by environmental conditions, too, they are not allowed to be kept at respective values obtained in advance, and it is necessary to frequently refresh the values to new, most suitable ones even during operation.
In Japanese Published Unexamined Patent Application No. Sho 63-184773, disclosed is a semiconductor laser drive device, which is equipped with a plurality of weighted current sources and a bias current source and, which is controlled by a microprocessor. In the publication, proposed is a system wherein, in determining the above-mentioned threshold current I.sub.th and inclination .eta., a bias current is first determined by detecting a slight emission from the laser when only the bias current is in an ON state, then a reference voltage for driving the plurality of current sources is set when a predetermined maximum quantity of emission is reached with increase in a drive current till a predetermined maximum quantity of emission under a condition of all the current sources being in ON states, and thereafter an intensity modulation between the light emission quantity by the bias current only and the maximum quantity of emission is effected by controlling a current source switch. When the system is applied to a semiconductor drive device in laser xerography, if the minimum light output is set to be too large like a current I.sub.3 shown in FIG. 7, a latent image is produced even with non-existence of data and on the other hand if it is set to be too small like a current I.sub.4 shown in FIG. 7 and a current is in the LED region, since a differential quantum efficiency is very low, a slight difference in level causes a large change in magnitude of the drive current, so that, in the case where a drive current is set at greatly small value, a current does not reach the laser oscillation region even if data is input and thereby almost no latent image is formed.
In Japanese Published Unexamined Patent Application No. Hei 4-122656, disclosed is a technique, in which beams with a plurality of predetermined light quantity levels P.sub.1, P.sub.2 are emitted in a region where a relation between the control current of a semiconductor laser and the optical output of the laser is linear, a plurality of control voltages to output a plurality of currents corresponding to the plurality of predetermined light quantities are stored and a relation between the current and emission power is obtained by a digital computing using a CPU. In Japanese Published Unexamined Patent Application No. Hei 4-320384, proposed is a technique in which a predetermined emission power is obtained by increasing the number of set levels of light quantities, even when a relation between the current supplied to a semiconductor laser and the emission power of the semiconductor laser is not linear. Though theses two examples are excellent in regard to accuracy of intensity modulation, a CPU is required because each conducts control in a digital mode. A data width required for obtaining 256 tones demands a data size of at least 8 bits, actually 12 bits due to non-linearity of characteristics of a photoreceptor, which is a cause of cost increase. In addition, a program of a great length has to be executed to process all data in a digital mode. For such reasons, APC consumes an excessively great length of time and as a result, if laser xerography is applied, a time to modulate a laser with a image data is restricted.
As a system implementing the same in an analog mode, Japanese Published Unexamined Patent Application No. Hei 6-334248 is named. In this publication, a system in which in order to conduct APC (Auto Power Control), the minimum value is first set in a sample hold circuit and then the maximum value is set in another sample circuit by controlling a gain of a D/A converter placed in a feedback loop for setting the maximum value. According to this system, since the setting of a control voltage is all effected in an analog mode, setting of the minimum or maximum value is effected when a signal is converged in a feedback loop, so that the setting is completed in a very short time. In this system, there is no need for a CPU, since no digital computing is performed. There is, however, a problem similar to the one in the technique disclosed in the publication of Unexamined Japanese Patent Application Sho 63-184773, that the minimum light quantity is set too large or too small.
There is a need for employing a region where the light quantity vs. the current is in a linear relation in order to effect intensity modulation with high accuracy. For that purpose, a laser oscillation threshold current I.sub.th which is an intersection point of an extrapolation of the straight line with the x axis (the axis of the current), where the light quantity is zero, should be determined (see [Heterostructures Lasers] HC CASEY, JR., M. B. PANISH, ACADEMIC PRESS NEW YORK 1978). However, since this I.sub.th has conventionally been able to be obtained only in a digital computing, a digital computing has had to be effected at the sacrifice of a speed APC and cost. If an analog computing had been adopted, a way would have been allowed, in which I.sub.th was computed in a simple manner with a very small quantity of light at the sacrifice of control accuracy without extrapolation, so that APC was conducted at a high speed and in a simple manner (see Japanese Published Unexamined Patent Application No. Sho 59-105568).